1. Field of the Disclosure
The present invention relates to fiber lasers. In particular, the invention relates to a laser module configured to prevent backreflected radiation at the desired wavelength from propagating towards and damaging laser diodes.
2. Discussion of the Prior Art
Solid and all fiber lasers use a semiconductor laser as a light source for optical pumping which is attractive because of its efficiency, reliability, compactness and acceptable beam quality. Where modest power is needed, in the order of a watt or so with superior beam quality, such as a diffraction-limited spot, optical fiber lasers with solid state pump sources using a single diode or a single diode array pump source have been used. For higher power applications where several watts may be needed, use has been made of laser diode arrays as pump sources coupled to special fiber gain structures. For even higher power requirements, ten watts or more, high power laser diode bars, whose cavities emit a few modes, may be coupled to such specialty gain fibers. In other words, high-power laser light can be obtained by combining the outputs from the emitting cavities of respective multiple laser diodes. Regardless of the required power, a pump source should exhibit high brightness and low power losses to have the desired beam quality.
The pump light, coupled to a gain block, is able to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium. The laser diodes are semiconductor devices containing a p-n junction which forms a diode,,and this junction functions as the active medium of the laser. The laser diodes are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 70 percent.
A typical laser assembly 10 is illustrated in FIG. 1 and includes a laser pump module 12 pumping a gain block 14. As readily realized by one of ordinary skills in the laser arts, light source module 12 may be easily associated with either the side- or end-pumping technique. The gain block 14 may include a single component, such as an oscillator provided with a gain medium which has a lasing cavity, or with multiple components, such as an oscillator and one or more amplifiers each pumped by respective pump modules 12. The pump module 12 may be configured with a single diode or an array of diodes 16, an output optical fiber 20 coupling the pump module to gain block 14 and coupling optics 18 guiding radiation from diodes 16 to fiber 20.
As a result of pumping, gain block 14 lases a light at the desired wavelength propagating along a waveguide. As the light propagates in a forward direction, the output light impinges upon a variety of internal and external obstacles or barriers. The internal barriers may include, for example, splices and interfaces between subsequent gain block components having different refractive indices. The external barriers may include the surface to be treated by the output light as, for example, in case of laser applications in welding or the cleaved or polished end of a fiber when the difference of refractive indices of air and glass causes light to reflect back. Up to several percents of the incident light, expressed in dB relative to incident power, tends to backreflect. The higher the incident power, the higher the backreflected power. The reflected light propagates backwards towards an upstream of waveguide terminating with a light source. A few dBs may detrimentally affect and even completely destroy laser diodes.
With the trend towards more and more powerful gain blocks, the protection of laser diodes from backreflection deserves a special attention. The backreflected light Lbr, at the lasing wavelength of gain block 14, is focused by optics 18 so that the only path it can further propagate along leads directly to laser diodes 16. This, in turn, may irreparably damage the diodes. As a consequence, the diodes need a protection.
As laser diodes become increasingly more powerful, so does the output light to be coupled into an active fiber of gain block 14 and so does the power of the output signal of block 14. At a certain level of power, the gain block/laser assembly is observed to have non-linear effects in the core of the active fiber. The detrimental effects of the non-linear effects are well known to one of ordinary skills and should be either minimized or completely avoided.
As readily realized by one of ordinary skills in the laser art, it is preferred that optically pumped gain blocks produce a substantially single transverse mode output. Diffraction limited, monomode output, optically pumped lasers, which use a simple, single pump source arrangement and are capable of producing monomode outputs around 100 mW, are useful for small-signal or low power applications. However, such lasers are not easily adapted to produce higher monomode powers, in the order of Watts and kilowatts.
While the above discussion is directed at the use of laser diode modules as a pump arrangement, it is apparent, that these modules may be used as a stand-alone unit for a variety applications. The backreflection phenomenon is always present regardless of any specific applications,
A need, therefore, exists for a robust high brightness module configured with means for selectively reflecting backreflected lights at respective wavelength or wavelengths so as minimize damage therefrom to one or multiple laser diodes.